Field of the Invention
The invention relates to an integrated memory having redundant units of memory cells, and a test method for its redundant units.
A memory having redundant columns is for example described in U.S. Pat. No. 4,485,459. In this case, the redundant columns are intended to replace, in terms of the respective address, a regular column of the memory. The address of the respective normal column to be replaced is stored by programmable elements in the form of interruptible electrical connections (fusible links or fuses). If the regular column to be replaced is subsequently addressed, one of the redundant columns is selected instead of the regular column to be replaced. Defects in the regular columns can thus be repaired.
It is expedient, in the case of a memory, also to test the memory cells of the redundant units (columns and/or rows of the memory) before regular units are replaced by redundant units. Otherwise, it can happen that a defective redundant unit is used when the repair is carried out. However, testing of the redundant units is made difficult by the fact that the programmable elements, which are usually embodied as fuses, can only be programmed once. They cannot, therefore, be programmed before a redundancy repair is actually carried out, in order to test the associated redundant units. On the other hand, as a rule all the redundant units are assigned programmable elements which are in the same programming state. This means that the fuses used are all intact and not interrupted. In many realizations the consequence of this is that when a specific address (e.g. the address 0) is applied, all the redundant units are addressed at once. This means that the same address in each case is assigned to these redundant units in the unprogrammed state of their programmable elements. In order to test each individual redundant unit, however, it is necessary that each redundant unit can be addressed individually. Otherwise, it is not possible to ascertain which of the tested redundant units is defective and which is not.